THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 97 



that all the blood will have traversed this chamber of the heart in this 

 time; part of the blood, on the shorter circuits, will have been through 

 more than once, and part will not yet have passed through. The time 

 required to handle this quantity of blood is much reduced when, as a 

 result of exercise or other factors, the heartbeat rate and the volume dis- 

 charged at each beat are increased. 



THE CLOTTING OF THE BLOOD 



As long as the circulatory system is uninjured and blood pressures 

 remain normal, the blood is a fluid that flows through even the smallest 

 capillaries. If pressure is reduced below a certain point the corpuscles 

 tend to settle out — a condition known as "sludged blood" —and clotting 

 may follow. If a blood vessel is cut or ruptured, the escaping blood 

 quickly clots, thus plugging the opened vessel and preventing continued 

 bleeding. Clotting may also occur at the sites of bacterial or mechanical 

 injury to the smooth inner wall of a blood vessel. 



In spite of our familiarity with the phenomenon of clotting and the fact that 

 it is a constant property of all normal blood, the mechanism responsible for its 

 occurrence is only partly understood. Nevertheless some of the important steps 

 in the process have been discovered. Consideration of these will emphasize two 

 important points: (1) what a large number of delicately and precisely adjusted 

 chemical reactions may be involved in an apparently simple physiological process; 

 and (2) how far we still are from a complete understanding of the way in which 

 the individual organism works. 



When we watch under the microscope a thin film of blood as it coagulates, 

 delicate threadlike strands can be seen to form within it. In larger quantities of 

 blood a dense, fine meshwork of such strands, or fibrils, develops, entangling the 

 blood cells, enclosing the liquid part of the blood, and transforming the whole 

 mass into a sticky, jellylike substance. Contraction of the network soon follows, 

 squeezing out of the clot a clear yellowish fluid called serum. (The main difference 

 between serum and blood plasma is that the former is no longer capable of clot- 

 ting). The fibrils of the network are made up of needlelike crystals of an insoluble 

 protein called fibrin. This substance is found to come from fibrinogen, one of the 

 characteristic soluble blood proteins always present in normal blood plasma. 

 Clotting, then, results from the change of fibrinogen to fibrin. What causes this 

 to happen when blood is shed, when blood vessels are injured, or when sludging 

 occurs? 



The full answer to this question cannot yet be given, but the known steps in 

 the process are approximately as follows: The reaction begins by the liberation 

 into the blood plasma of a substance called prothrombokinase, which is present in 

 most cells but has its chief source in the blood platelets, which break down and 

 free it under conditions presently to be described. In the presence of calcium 

 ions, which are of normal occurrence in blood plasma, prothrombokinase is 

 changed to the active enzyme thrombokinase. Plasma contains another enzyme 



